sdm845: Implement bitbang UART for bootblock

This patch replaces the UART in the bootblock of SDM845 with a bitbang implementation. Since SDM845 hardware UART needs a firmware blob loaded into it before it becomes usable, it is not really suited for use in the bootblock (since by the time we can read blobs from SPI, the bootblock is essentially over anyway). This solution allows us to still have some console output during early SoC initialization.